Retroreflective devices are designed to reflect radiation back towards its source, and such devices are thus frequently used to return radiation toward radiating optical sources when it is inconvenient or undesirable to actively generate radiation at locations, remote from the optical sources, which need to send response radiation to fixed or mobile base locations at which the radiating optical sources are sited. Common examples include the use of special reflective materials for safety clothing or signage, cat's eye markers in road surfaces, and measurement points in land surveying or robotic machinery. Retroreflective devices may also be used in combination with optical modulation mechanisms in order to establish two-way optical communication between the base station and the remote location without needing an optical source at the remote end of the link.
Retroreflective devices currently in common use can be classified into two main types.
The first of these accomplishes retroreflection without any focusing of the incoming radiation from the source. This type is commonly available as a component with a set of three mutually perpendicular reflecting surfaces, and is known as a “corner-cube” retroreflector. Different embodiments of this type of reflector exist, but all require very high quality in the reflecting surfaces, which also need to be very accurately assembled in order to provide precise angular coincidence between the incident and reflected beams. Consequently, such components are expensive to make, but they have the advantage of providing diffraction limited performance which gives excellent quality to the reflected beam. Disadvantageously, however, such devices exhibit a limited field of view which varies with the construction details, but which cannot allow coverage of arbitrary angles of incidence across a full hemisphere without grouping several reflectors together, where each reflector is pointed in a different direction. This increases the expense and the complexity of constructing such devices with high field of view.
The second type of retroreflective device currently in common use employs focusing of the incident radiation onto a primary reflecting surface. This type is known as a “cat's eye” retroreflector, and commonly employs glass spheres, or cemented hemispheres, in order to provide retroreflection for paraxial incident rays. Such devices can be made very small (for example with sub-millimeter diameters) and offer a very wide field of view, including a complete hemisphere or more in a single component. Furthermore, single spheres can be manufactured in quantity at low cost. The main disadvantage of this design is that the reflected radiation is subject to severe spherical aberration for non-paraxial rays, and this can strongly reduce the far-field intensity of the reflected beam measured on-axis. It also leads to significant beam divergence, making the reflection visible far from the axis, which can be undesirable in some applications, for example in free-space communication where privacy is desired.
A class of lenses, called ‘graded refractive index’ (or GRIN) lenses, is known, in which the material of the lens exhibits gradual variations in refractive index through its volume. An example is the so-called “GRIN-rod” lens, which is a graded-index lens with cylindrical symmetry and radial parabolic index distribution. See S. Nemoto and J. Kida, ‘Retroreflector using gradient-index rods’ Appl. Opt. 30(7), 1 Mar. 1991, p. 815–822.
In a publication entitled “Gradient Index Optics” published by Academic Press in 1978, E. W. Marchand describes at pages 2 and 3 a lens, previously discovered by Luneburg, having an index function with spherical symmetry about a point. Marchand describes the Luneburg lens as difficult to make (at least for radiation in the visible region of the spectrum), and goes on to say that the lens, even if it can be made, has limited possibilities for useful application, though he does suggest a possible modification of the lens, incorporating a mirror to produce an action similar to that of a corner cube.
Sphere lenses with refractive index distributions possessing spherical symmetry are known as ‘GRIN-sphere’ lenses, having a spherically symmetric refractive index distribution in which the refractive index varies gradually across a radial cross-section. Such lenses are known to exhibit improved spherical aberration compared to uniform sphere lenses. See Y. Koike, A. Kanemitsu, Y. Shioda, E. Nihei and Y. Ohtsuka, ‘Spherical gradient-index polymer lens with low spherical aberration’ Appl. Opt. 33(17), 1 Jun. 1994, p. 3394–3400.
GRIN-rod lenses have been proposed for use in retroreflective devices, but these lenses suffer from restrictions on field of view similar to those exhibited by corner-cube retroreflectors. Other types of cat's eye retroreflector can be based on catadioptric lens designs, but these also share the aforementioned restrictions on field of view.